This project concerns the interaction of specifically sensitized spleen lymphocytes and tumor cells. Efforts were focused on correlating the results of in vivo cytotoxicity tests with in vivo tests of tumor cell killing. The results indicate that the cell populations responsible for in vitro cytotoxicity are separable from the cell population responsible for adoptive transfer of immunity. These findings indicate the need for confirmation of results of in vitro experiments in animals. Cells mediating adoptive transfer of immunity do react with the targets in vitro, but this interaction does not lead to target cell destruction.